The irreversible neurological deficits in multiple sclerosis (MS) and its animal model, experimental allergic encephalomyelitis (EAE), are due primarily to axonal and neuronal degeneration brought on by a pro-inflammatory assault on the central nervous system (CNS) by autoreactive T cells and other immune cells. Previous studies in our laboratory and others have indicated that the calcium (Ca2+)-activated protease, calpain, plays a role in both the immune arm and the neurodegenerative arm of EAE develop- ment. The goal of this project is to understand the precise timing and molecular mechanisms involved in axonal and neuronal damage in acute and chronic EAE as these events correlate with increased calpain expression and activity and to examine if inhibiting calpain activity will attenuate disease progression by blocking components of the immune arm and/or neurodegenerative arm. Thus, we hypothesize that changes in Ca2* influx and calpain activation will promote infiltration of autoreactive T cells and macrophages, leading to axonal and neuronal degeneration. A corollary hypothesis is that calpain inhibition will prevent migration of activated T cells and macrophages, reduce the production of antigenic myelin basic protein (MBP) peptides, and delay or prevent axonal and neuronal degener- ation; which will lead to reduced disability (paralysis, limp tail) in acute and relapsing/remitting (R/R) EAE models. Preliminary data indicate that Ca2+ influx, calpain activation, T cell activation, axonal degeneration, and apoptotic proteins are increased in EAE spinal cord at the onset of acute disease. Additionally, treatment with the calpain inhibitor SJA6017 (SJA) reduced immune cell activation and infiltration, as well as calpain activity and expression, and prevented cell death in EAE spinal cord, as compared to vehicle-treated animals. Calpain inhibition also reduced MBP degradation by MBP-specific T cells in vitro and greatly attenuated disease development in a R/R adoptive transfer EAE mouse model. To further investigate the roles of calpain in EAE, the following Specific Aims will be addressed: (1) Characterize the timing of Ca2+ influx and calpain activation as these events correlate with increased mechanisms of axonal damage and apoptosis in neurons and glial cells in spinal cord from Lewis rats with acute EAE. (2) Investigate whether treatment with the calpain inhibitor SJA will attenuate EAE development in the acute EAE Lewis rat model. (3) Examine whether calpain inhibition by SJA treatment will delay or block disease development and attenuate neurodegeneration in the R/R model of EAE in SJL/J mice. The proposed studies targeting calpain as therapeutic strategy for attenuating clinical disability in EAE/MS by blocking immune dysfunction and/or neurodegeneration will further development of novel treatments for these debilitating diseases.